In accordance with the teachings of U.S. Pat. No. 3,266,302, a measurement vehicle is propelled over a road surface, and surface profile (W) is measured as a conjoint function of displacement of the vehicle suspension system (W-Y) and the twice-integrated output (Y) of an accelerometer carried by the vehicle. The disclosed system is described as effecting profile measurement with respect to a plane of reference, defined by inertia of the vehicle, over a total frequency range of road surface undulations determined at lower frequencies by accelerometer response characteristics and at higher frequencies by the vehicle suspension displacement transducer. However, the signal/noise response capabilities of the accelerometer at lower frequencies and any steady-state offset in the electrical output of both transducers, coupled with the described double integration, limit the capabilities of the system to the extent disclosed in the referenced patent.
To overcome the low frequency response difficulties which inhere in the accelerometer/double-integration technique, it was proposed in Spangler et al, "A Method of Measuring Road Profile", GMR-452, General Motors Corp. (1964) to subject the accelerometer output to a time domain highpass filter for attenuating the low frequency response prior to time domain double integration. However, since the spatial frequency content of the road profile varies in proportion to vehicle velocity, the provision of the highpass time domain filter causes the measured profile to vary as a function of vehicle speed. This problem was alleviated to some extent by providing a highpass filter with a step-wise variable natural or cutoff frequency thereby to accommodate step-wise differing but constant vehicle speed.
A further improvement which is prior art to the present invention implemented time domain digital processing techniques in place of, but exactly analogous to, the analog time domain processing techniques proposed in the above-referenced patent and GMR paper. This improvement embodied the capacity for user-input of desired frequency (or wavelength) measuring capability and contemplated vehicle speed. A corresponding highpass filter natural frequency was computed and applied to the accelerometer output in time domain computation of road profile. Although the technique so implemented effectively replaces the earlier step-wise selectable filter with a continuously variable filter, it was still necessary to maintain a constant vehicle velocity during the measurement process.